1. Field of Invention
The present invention relates to an acoustic transducer device for noise processing, and more particularly to an acoustic transducer device for noise processing capable of switching between a feed-forward noise cancellation mode and a feed-back noise cancellation mode.
2. Related Art
People are apt to be fretful when they are affected by noises. If a person has been under a noisy environment for a long time, a permanent hearing impairment may even be caused. Therefore, in recent years, technologies for cancelling noise are continuously proposed. In the field of earphones, early noise cancellation technology is based on structural improvements. For example, ear covers or ear muffs with a good sound isolation effect are selected. Generally, such earphones are capable of isolating noises above 800 Hz, but have a poor sound isolation effect against noises below 800 Hz, especially low frequency noises. Hence, such a technology that is commonly called “passive noise cancellation” cannot perfectly solve the noise problem completely. For this reason, an electronic noise cancellation technology called “active noise cancellation” is frequently proposed recently in order to eliminate the deficiencies of “passive noise cancellation”. The “active noise cancellation” technology may be classified into the following two types: feed-forward noise cancellation technology and feed-back noise cancellation technology.
FIG. 1A is a schematic structural view of a feed-forward noise cancellation earphone 1. Referring to FIG. 1A, the earphone 1 is provided with a microphone 10, a noise cancellation circuit 11, and a speaker 12. The speaker 12 faces the ear canal of a user. After the microphone 10 receives an external noise, the noise cancellation circuit 11 generates an anti-noise signal to cancel the noise received in the earphone 1. The advantage of this implementation lies in that, the microphone 10 receives only the noise and does not receive any sound output by the speaker 12, so that an open-loop system is formed, and no closed-loop oscillation or echo will be caused. Thus, the circuit may be adjusted to the best noise cancellation effect independently. However, since the noise undergoes a plurality of reflections when passing through ears of a user and the amplitude and phase of the noise have changed, the noise received by the microphone 10 is quite different from that within the ears of the user. Moreover, since the external noise is highly directional, it is difficult to meet noise cancellation requirements against noises from different directions by using a single circuit.
FIG. 1B is a schematic structural view of a feed-back noise cancellation earphone 2. Referring to FIG. 1B, the earphone 2 is also provided with a microphone 20, a noise cancellation circuit 21, and a speaker 22. The speaker 22 faces the ear canal of a user. The microphone 20 is disposed between the speaker 22 and the ear canal, so that the noise received from the ear by the microphone 20 is the same as that heard by the user. After the noise is filtered, amplified, and inverted in phase by the noise cancellation circuit 21, the speaker 22 is driven to produce a sound. In such a closed-loop system design, the microphone 20 is insensitive to the direction of the noise, and a sound with a high signal-to-noise ratio may be generated after a feedback signal and a sound signal are superposed, so the noise is the lowest when transmitted to and heard by the ear of the user. Although the feed-back noise cancellation earphone has a good noise cancellation effect, resonance attenuation occurs in a high frequency range. As a result, for users that usually use the earphone to listen to music, the earphone with the noise cancellation function undesirably compromises the effect of the original sound output, and thus fails to achieve a desirable performance.